The present disclosure relates to prosthetic heart valves. More particularly, it relates to prosthetic heart valves appropriate for transcatheter delivery to a target site and adjustable to a shape of the target site, such as a native mitral valve.
A human heart includes four heart valves that determine the pathway of blood flow through the heart: the mitral valve, the tricuspid valve, the aortic valve, and the pulmonary valve. The mitral and tricuspid valves are atrio-ventricular valves, which are between the atria and the ventricles, while the aortic and pulmonary valves are semilunar valves, which are in the arteries leaving the heart. The tricuspid valve, also known as the right atrio-ventricular valve, is a tri-flap valve located between the right atrium and the right ventricle. The mitral valve, also known as the bicuspid or left atrio-ventricular valve, is a dual-flap valve located between the left atrium and the left ventricle.
As with other valves of the heart, the mitral valve is a passive structure in that it does not itself expend any energy and does not perform any active contractile function. The mitral valve includes an annulus that provides attachment for the two leaflets (anterior leaflet and posterior leaflet) that each open and close in response to differential pressures on either side of the valve. The leaflets of the mitral valve are dissimilarly shaped. The anterior leaflet is more firmly attached to the annulus, and is somewhat stiffer than the posterior leaflet (that is otherwise attached to the more mobile posterior lateral mitral annulus). The anterior leaflet protects approximately two-thirds of the valve. The anterior leaflet takes up a larger part of the annulus and is generally considered to be “larger” than the posterior leaflet (although the posterior leaflet has a larger surface area). In a healthy mitral valve, then, the anterior and posterior leaflets are asymmetric.
Ideally, the leaflets of a heart valve move apart from each other when the valve is in an open position, and meet or “coapt” when the valve is in a closed position. Problems that may develop with valves include stenosis in which a valve does not open properly, and/or insufficiency or regurgitation in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve. The effects of valvular dysfunction vary, with regurgitation or backflow typically having relatively severe physiological consequences to the patient.
Diseased or otherwise deficient heart valves can be repaired or replaced using a variety of different types of heart valve surgeries. One conventional technique involves an open-heart surgical approach that is conducted under general anesthesia, during which the heart is stopped and blood flow is controlled by a heart-lung bypass machine.
More recently, minimally invasive approaches have been developed to facilitate catheter-based implantation of the valve prosthesis on the beating heart, intending to obviate the need for the use of classical sternotomy and cardiopulmonary bypass. In general terms, an expandable prosthetic valve is compressed about or within a catheter, inserted inside a body lumen of the patient, such as the femoral artery, and delivered to a desired location in the heart.
The heart valve prosthesis employed with catheter-based, or transcatheter, procedures generally includes an expandable multi-level frame or stent that supports a valve structure having a plurality of leaflets. The frame can be contracted during percutaneous transluminal delivery, and expanded upon deployment at or within the native valve. One type of valve stent can be initially provided in an expanded or uncrimped condition, then crimped or compressed about a balloon portion of a catheter. The balloon is subsequently inflated to expand and deploy the prosthetic heart valve. With other stented prosthetic heart valve designs, the stent frame is formed to be self-expanding. With these systems, the valved stent is crimped down to a desired size and held in that compressed state within a sheath for transluminal delivery. Retracting the sheath from this valved stent allows the stent to self-expand to a larger diameter, fixating at the native valve site. In more general terms, then, once the prosthetic valve is positioned at the treatment site, for instance within an incompetent native valve, the stent frame structure may be expanded to hold the prosthetic valve firmly in place. One example of a stented prosthetic valve is disclosed in U.S. Pat. No. 5,957,949 to Leonhardt et al., which is incorporated by reference herein in its entirety.
The actual shape and configuration of any particular transcatheter prosthetic heart valve is dependent, at least to some extent, upon the valve being replaced or repaired (i.e., mitral valve, tricuspid valve, aortic valve, or pulmonary valve). The stent frame must oftentimes provide and maintain (e.g., elevated hoop strength and resistance to radially compressive forces) a relatively complex shape in order to achieve desired fixation with the corresponding native anatomy. Moreover, the stent frame must have a robust design capable of traversing the tortuous path leading to the native valve annulus site. These constraints, as well as other delivery obstacles such as difficulties in precisely locating and rotationally orienting the prosthetic valve relative to the native annulus, dictate that in some instances, it is beneficial to utilize a valve structure with the prosthesis that purposefully differs from the native (healthy) valve structure or leaflets. For example, the transcatheter prosthetic heart valve may incorporate a symmetric valve structure for replacing an asymmetric native valve, an asymmetric valve structure to replace a symmetric native valve, a valve structure having a greater or lesser number of leaflets than the native valve, a valve structure having leaflet shapes differing from the native valve, etc. These same design choices can arise in the context of other intraluminal prosthetic valves.
While the so-configured prosthetic valve can thus achieve the intended benefits (such as ease of implant) along with long term viable performance as a functioning valve, in some instances other concerns may arise. For example, challenges remain for preventing movement and/or migration of the prosthetic valve that could occur during the cardiac cycle and to prevent leakage between the implanted prosthetic valve and the surrounding tissue (paravalvular leakage). Further, the anchoring forces generated by a transcatheter prosthetic heart valve upon deployment may lead to interference with flow or conduction channels within the heart. For example, the design of a prosthetic mitral valve must account for the elliptical shape, dynamic nature and complex subvalvular apparatus associated with the native mitral valve and proximate anatomy. The requisite radial forces generated by many transcatheter prosthetic mitral valve designs for achieving desired anchoring with this native mitral valve anatomy can lead to obstruction of the left ventricular outflow tract (LVOT) and/or negatively impact the coronary sinus.